A shiftable friction clutch of this type is known in which rolling members are arranged between the inclined surfaces which are provided on the friction ring and the inclined surfaces which are provided on the shaft (German Pat. No. 20 33 772). In such a clutch, the shifting operation is initiated only by a relatively small force, the further shifting operation being carried out automatically until completion without that one would have to or could controllingly interfere externally in any manner. In other words: the shifting operation is only started externally, the further course occurs uncontrollably. Here exists--in contrast to a hydraulically or pneumatically shifted clutch for example--no possibility to reduce or even to avoid the impacts or impulses which can often be observed at the end of a shifting operation as a consequence of a sudden rise in the transmitted torque. In some cases this so-called engaging force or surge can be accepted, however, it is many times felt as interfering, for example, in drives for sport and recreational boats.
Other types of known structures are disclosed in U.S. Pat. Nos. 3,203,525 and 3,703,226 and Application Ser. No. 041,923, filed May 24, 1979 and assigned to the same assignee as is the present invention.
Therefore, the basic purpose of the invention is to prevent or at least considerably reduce in a friction clutch which is operated through a servo force the impact or surges which occur at the end of the engaging operation. As few as possible structural changes are thereby required in the conventional types of construction of clutches.